Distributed amplification is the amplification of portions of a signal along the length of a first (input) transmission line by either discrete active elements (amplifiers) or microwave/milometer-wave integrated circuit technology (MIMIC), the outputs of which are combined along a second (output) transmission line to produce the resultant amplified signal.
Conventional distributed amplifiers (CDAs) are commonly used for example in ultra-broadband communications and radio imaging systems due to their inherent high gain bandwidth properties which arise due to high cut off frequency associated with the transmission line structure of amplifier. CDAs have been widely and successfully investigated and implemented for more than three decades in various applications for both hybrid and monolithic technologies. Due to flat gain, uniform group delay and low noise figure over a large frequency band, distributed amplifiers (DA) are a good candidate for ultra-broadband low noise amplifiers in for example millimeter wave receivers. Theoretically by using a properly designed transmission line, infinite bandwidth can obtain with no limitation on gain. However, due to loss associated with transmission line implemented in a CDA structure, bandwidth is limited to cut off frequency of the transmission line formed by inductors and parasitic capacitors of transistors and pieces of microstrip or a coplanar waveguide. FIG. 1 shows a representative schematic of a CDA.
By proper choice of gate and drain inductors one can derive:
                                                        L              g                                      C              gs                                      =                                                            L                d                                                              C                  ds                                +                                  C                  a                                                              =                      Z            0                                              (        1        )            where Z0 is transmission line characteristic impedance which is usually 50Ω, Cgs and Cds are gate-to-source and drain-to-source capacitances and Ca is a shunt capacitance connected to the drain of each transistor to synchronize the gate and drain transmission lines. For signals amplified by each transistor arrive in phase to output, the phase velocities of gate and drain line should be equal. This means:√{square root over (LgCgs)}=√{square root over (Ld(Cds+Ca))}  (2)
From (1) and (2), we can conclude that Lg=Ld. The signals travelling in left direction in the drain line are out of phase and cancel out or are absorbed in termination impedance of Rd=Z0. In the gate line, the signal is absorbed in the gate termination impedance when Rg=Z0. The resulting amplifier has low pass frequency response.
The cut off frequency of CDA, can be expressed as:
      f    c    =            1              π        ⁢                                            L              g                        ⁢                          C              gs                                            =          1              π        ⁢                                  ⁢                  Z          0                ⁢                  C          gs                    
The main disadvantage of CDA is low frequency range of operation due to the cut-off frequency of the transmission line associated with the amplifier.
There is a need for a distributed amplifier design, and distributed amplifiers based on such design, that address the aforesaid disadvantages.